Effect of Threonine on the Toxicity of Excess Tyrosine and Cataract Formation in the Rat

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Abstract
Typical symptoms, i.e., eye and paw lesions, of tyrosine toxicity were found when 3% L-tyrosine was included in a 6% casein diet fed ad libitum or when 5% L-tyrosine was included in a 6% casein diet fed only 2 hours each day. The addition of 0.8 to 1.25% of L-threonine to the high tyrosine diet prevented the appearance of the eye and paw lesions and improved the growth. However, when the diet containing high tyrosine and high threonine was fed for over 2 months, cataracts were observed in 80% of the rats. Growth was improved and the cataracts were completely prevented by a supplement of 0.2% tryptophan; although the eye and paw lesions caused by the high tyrosine were still observed when 0.2% tryptophan was added to the 3% tyrosine diet. It appears that the addition of 1.25% L-threonine to the 3% tyrosine diet increases the tryptophan requirement.